1. Field of the Invention
The present invention generally relates to earth-boring drill bits and other tools for drilling subterranean formations, and to methods of designing and fabricating such earth-boring drill bits. More particularly, the present invention relates to earth-boring drill bits and other tools for drilling subterranean formations that exhibit predictable walk characteristics, as well as to methods for designing and fabricating the same. Furthermore, the present invention relates to systems and methods for collecting data relating to imbalance forces and walk characteristics of earth-boring drill bits and other tools for drilling subterranean formations.
2. State of the Art
Rotary drill bits are commonly used for drilling bore holes or well bores in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. Generally, the cutting elements of a fixed-cutter type drill bit have either a disk shape or a substantially cylindrical shape. A cutting surface comprising a hard, superabrasive material, such as mutually bound particles of diamond, may be provided on a substantially circular end surface of each cutting element. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutters. Typically, the cutting elements are fabricated separately from the bit body and secured within pockets formed in the outer surface of the bit body. A bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements to the bit body. The fixed-cutter drill bit may be placed in a bore hole such that the cutting elements are adjacent the earth formation to be drilled. As the drill bit is rotated, the cutting elements scrape across and shear away the surface of the underlying formation.
The bit body of a rotary drill bit typically is secured to a hardened steel shank having an American Petroleum Institute (API) thread connection for attaching the drill bit to a drill string. The drill string includes tubular pipe and equipment segments coupled end to end between the drill bit and other drilling equipment at the surface. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit within the bore hole. Alternatively, the shank of the drill bit may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit.
The bit body of a rotary drill bit may be formed from steel. Alternatively, the bit body may be formed from a particle-matrix composite material. Such bit bodies typically are formed by embedding a steel blank in a carbide particulate material volume, such as particles of tungsten carbide (WC), and infiltrating the particulate carbide material with a liquified metal material (often referred to as a “binder” material), such as a copper alloy, to provide a bit body substantially formed from a particle-matrix composite material. Drill bits that have a bit body formed from such a particle-matrix composite material may exhibit increased erosion and wear resistance relative to drill bits having steel bit bodies.
The process of drilling a subterranean formation is often a three-dimensional process, as the drill bit not only penetrates the formation linearly along a vertical axis, but is either purposefully or unintentionally drilled along a curved path or at an angle relative to a theoretical vertical axis extending into the subterranean formation in a direction substantially parallel to the gravitational field of the earth. The term “directional drilling,” as used herein, means both the process of directing a drill bit along some desired trajectory through a subterranean formation to a predetermined target location to form a bore hole, and the process of directing a drill bit along a predefined trajectory in a direction other than directly downwards into a subterranean formation in a direction substantially parallel to the gravitational field of the earth to either a known or unknown target. Referring to FIG. 1, the orientation of a drill bit and/or a well bore hole may be described in terms of an “inclination angle” and a “direction angle” using a theoretical vertical axis 2 extending into the ground and oriented parallel to the gravitational field of the earth, and a horizontal plane 4 oriented substantially perpendicular to the theoretical vertical axis 2. The inclination angle may be defined as the shortest angle between the longitudinal axis 6 extending through the bit and/or the well bore hole and the theoretical vertical axis 2. The direction angle may be defined as the angle extending in the horizontal plane 4 from a reference direction, such as North, in the clockwise direction to the projection 8 of the longitudinal axis 6 extending through the bit and/or the well bore hole onto the horizontal plane 4. The direction angle is often referred to in the art as the “azimuth” or the “azimuthal angle.”
As an example, when a well bore hole extends substantially vertically downward into the subterranean formation, the inclination angle is zero and there is no direction angle. Furthermore, when a well bore hole extends substantially horizontally in a lateral direction within a subterranean formation, the inclination angle is about ninety degrees, and the direction angle may be any angle between zero and three hundred sixty degrees.
Several approaches have been developed for directional drilling. For example, it is known to use a bottom hole assembly (BHA) that includes a motor driven by a flow of drilling fluid, or “mud” pumped down the drill string to the motor for rotating the drill bit as mounted to a bent sub or a bent housing for orienting the drill bit at an angle with respect to the bore hole. Other approaches involve, for example, the use of a “whipstock,” which may include a wedge-shaped tool positioned at the bottom of the well bore hole and oriented to deflect the drill bit at an angle with respect to the longitudinal axis of the bore hole and drill through a side wall thereof. Yet another method for directional drilling involves the use of a “jetting bit,” which may include at least one drilling fluid nozzle configured to orient a jet of fluid emitted thereby in a predetermined direction relative to the bit face. The drill bit may be positioned at the bottom of the bore hole in a desired orientation, and the jet of fluid emitted from the nozzle is used to erode a pocket out of the formation material surrounding the bore hole while the drill bit is not rotating. The drill bit may then be advanced into the eroded pocket, and rotation of the drill bit is resumed, the drill bit advancing at an angle relative to the prior trajectory.
After a target within a subterranean formation has been identified, a trajectory for a drill bit and the well bore hole produced thereby may be predefined. The term “deviation control,” as used herein, means the process of maintaining the drill bit, and thus the well bore hole, within predetermined limits relative to a predefined trajectory.
The processes of directional drilling and deviation control are complicated by the complex interaction of forces between the drill bit and the walls of the subterranean formation lining the well bore hole.
In drilling with rotary drill bits and, particularly with fixed-cutter type rotary drill bits, it is known that if a lateral force (often referred to as a side force or a radial force) is applied to the drill bit, the drill bit may “walk” or “drift” from the straight path that is parallel to the intended longitudinal axis of the well bore hole. When a drill bit walks in such a way that the direction angle increases (increasing azimuth), the drill bit may be said to walk to the right or to exhibit “right walk.” Similarly, when a drill bit walks in such a way that the direction angle decreases (decreasing azimuth), the drill bit may be said to walk to the left or to exhibit “left walk.” When a drill bit does not walk or drift away from the straight path that is parallel to the longitudinal axis of the well bore hole at the bottom thereof, the bit may be referred to as an “anti-walk” drill bit and may be said to exhibit “neutral walk.”
In a similar manner, when a drill bit drifts in a direction such that the inclination angle increases, the drill bit is said to exhibit a tendency to “build,” and when a drill bit drifts in a direction such that the inclination angle decreases, the drill bit is said to exhibit a tendency to “drop.” Drill bits may, however, exhibit a tendency to walk to the right or to the left more often than they exhibit a tendency to build or drop.
Many factors or variables may at least partially contribute to the reactive forces and torques applied to a drill bit by the surrounding subterranean formation. Such factors and variables may include, for example, the “weight on bit” (WOB), the rotational speed of the bit, the physical properties and characteristics of the subterranean formation being drilled, the hydrodynamics of the drilling fluid, the length and configuration of the bottom hole assembly (BHA) to which the bit is mounted, and various design factors of the drill bit including the cutting element size, radial placement, back (or forward) rake, side rake, etc. Various complex modeling and computational methods known in the art may be used to calculate the forces and torques acting on a drill bit under predetermined conditions and parameters.
In view of the above, it has been suggested in the art to design fixed-cutter type rotary drill bits that exhibit predetermined walk characteristics (i.e., left walk, right walk, or neutral walk) using these complex modeling and computational methods. For example, a drill bit design may be created using three-dimensional modeling software. The design variables (together with other variables relating to the anticipated drilling conditions such as those listed above) may then be used by computational software to estimate by mathematical calculations the reactive forces and torques applied to the drill bit by the surrounding subterranean formation during drilling, and these forces and torques may be used to estimate the trajectory of the drill bit through the subterranean formation.
Such efforts have been met with limited success. This may be due, at least in part, to the inability to fabricate drill bits according to the exact dimensions specified in the drill bit design. For example, the cutting elements of a fixed-cutter type rotary drag bit are often hand-brazed into cutter pockets on the face of the drill bit, and even slight variations in cutter position (back rake angle, side rake angle, etc.) may cause a drill bit to exhibit unexpected walk behavior. For example, a drill bit design may be created and configured to exhibit predetermined walk characteristics. Several drill bits may be fabricated according to the single drill bit design within manufacturing tolerances. In the field, however, some of these drill bits may exhibit left hand walk, others may exhibit right hand walk, and still others may exhibit neutral walk.
In view of the above, there is a need in the art for methods for designing and fabricating rotary drill bits for drilling subterranean earth formations that exhibit predictable walk characteristics.